The present invention relates to a film forming method and a film forming apparatus used in a semiconductor fabrication process.
A semiconductor device fabrication process includes a photolithography step of performing resist coating, exposure, and development for an object to be processed, e.g., a semiconductor wafer (to be referred to as a wafer hereinafter) such as a silicon substrate. More specifically, a wafer is coated with a coating solution such as a photoresist solution to form a photoresist film. The photoresist film is exposed by projecting a pattern such as a circuit in a reduced scale by using a photographic technology, and the exposed photoresist film is developed.
The above photolithography step is crucial in increasing the packing density of semiconductor devices. To evenly form a resist film on the surface of a wafer in this photolithography step, a solvent of a resist solution is dropped on the surface of the wafer before coating of the resist solution and spread on the wafer surface by rotating the wafer. Thereafter, the resist solution is dropped on the surface of the rotating wafer, thereby spreading out the resist solution by making the resist solution follow the solvent. This method is disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 61-91655, 61-150332, and 7-320999.
In the conventional coating film forming methods of the above sort, however, a certain volume of a resist solution A must be supplied, as shown in FIG. 1A, to make the resist solution cover the edge of a wafer. Therefore, as shown in FIGS. 1A to 1C, it is possible that the end portion of the resist solution A sets before the resist solution A covers the edge of a wafer W. Consequently, not only the film thickness becomes nonuniform but the amount of resist solution cannot be decreased.
To avoid the setting of the end portion of the resist solution A, the rotating speed of a wafer can be increased. However, if the wafer W is rotated at a high speed, the peripheral speed of the outermost periphery of the wafer W increases. When the peripheral speed exceeds a certain speed, evaporation of a solvent such as a thinner in the resist solution A is disturbed by turbulence of an airstream above the perimeter of the wafer W, as a parameter other than the moment by which the resist solution A spreads using a centrifugal force. This results in vertical stripes on the edge of the wafer W. As a consequence, even coating of the resist solution A becomes difficult. To remove the unevenness on the edge of the wafer W, the use amount of resist solution must be increased.
In particular, since 8" and 12" wafers are presently used as the packing density of semiconductor devices is increased, it is necessary to decrease the rotating speed of a wafer compared to that of a 6" wafer. This increases the use amount of resist solution and makes a uniform resist film thickness difficult to obtain.
As a method of forming a resist film on the surface of a wafer in the photolithography step, a spin coating method is known in which a resist solution is dropped on the surface of a wafer so held as to be horizontally rotatable by a spin chuck and the wafer is rotated to coat the wafer surface with the resist solution. When a resist solution having a predetermined viscosity is used in this coating method, the film thickness of the resist film can be changed by changing the rotating speed of a wafer. That is, the film thickness can be decreased by increasing the wafer rotating speed and increased by decreasing the rotating speed.
With the recent increase in the packing density of semiconductor devices, conventional 6" wafers tend to be replaced with large-diameter wafers such as 8" and 12" wafers, and a similar coating method is also used for these large-diameter wafers.
Unfortunately, when such a large-diameter wafer is rotated at a high speed, turbulent air is produced above the edge of the wafer as described above, and this makes a resist film uneven on the edge of the wafer. To form a resist film on a large-diameter wafer, therefore, a resist solution with a low viscosity must be used. Also, to form resist films differing in film thickness, it is necessary to prepare a plurality of different types of resist solutions of each different viscosity and perform coating by changing the resist solutions each time the types of resist films to be formed are changed. Consequently, the productivity decreases in this method of performing coating while changing different types of resist solutions differing in viscosity. Additionally, the maintenance of these resist solutions is troublesome and the storage space of the solutions needs to be secured.